1. Field of Invention
This invention relates to methods and apparatus for heat treatment of semiconductor films provided over thermally sensitive substrates. In particular, the invention relates to the formation of polycrystalline silicon thin-film transistors on glass substrates for use in liquid crystal displays (LCDs) or organic light emitting diode displays (OLEDs).
2. Related Art
Active matrix liquid crystal displays (LCDs) and organic light emitting diode displays (OLEDS) use active matrix circuits having thin film transistors (TFTs) provided on a glass substrate. There are also proposals to form the active plate of such devices on plastic substrates.
Active matrix LCDs typically use amorphous silicon (a-Si) TFTs, although there has been much recent interest and development of TFTs using polycrystalline silicon (poly-Si). This technology can give superior image resolution, and can also permit integration of peripheral drive circuits onto the substrate of the pixel array. For current-addressed displays, such as OLEDs, poly-Si TFTs provide better current driving capabilities.
The main difficulty in the fabrication of poly-Si devices on current glass substrates is the heat treatment method for forming the crystallized semiconductor layer from a deposited amorphous silicon layer, and the activation of implanted dopants. Glass is easily deformed when exposed to temperatures above 500 degrees Celsius for substantial lengths of time.
Various heat treatment methods have been developed for the crystallization of a deposited amorphous silicon layer.
Solid phase crystallization (SPC) is one common method for crystallizing amorphous silicon. In this process, the amorphous silicon is subject to heat treatments at temperatures approaching 600 degrees Celsius for a period of several hours. This method can lead to damage of the glass substrate. In particular, if low cost glass substrates are to be used, the temperature is too high to be used or else, if the annealing is done at relatively low temperatures, the anneal time is too long.
Laser treatment methods can operate at lower temperatures, for example excimer laser crystallization and metal-induced crystallization. Excimer laser crystallization uses nano-second laser pulses to melt and solidify the amorphous silicon into a crystalline form. Theoretically, this offers the possibility of annealing the amorphous silicon at its optimum temperature without degrading the glass substrate upon which it is mounted. However, this method has critical drawbacks for its use in mass production. The grain structure of poly-Si film through this process is extremely sensitive to the laser beam energy, so that a uniformity in grain structure and hence the device characteristics can not be achieved. Also, the beam size of the laser is relatively small. The small beam size requires multiple laser passes, or shots, to complete the crystallization processes for large size glass. Since it is difficult to control precisely the laser, the multiple shots introduce non-uniformities into the crystallization process.
The metal induced crystallization process involves addition of various metal elements such as Ni, Pd, Au, Ag, and Cu onto amorphous silicon films in order to enhance the crystallization kinetics. Use of this method enables crystallization at low temperatures below 600 degrees Celsius. This method, however, is limited by poor crystalline quality of poly-Si and metal contamination. The metal contamination causes a detrimental leakage current in the operation of poly-Si TFTs. Another problem of this method is the formation of metal silicides during the process.
Heat treatment is also used for activation of dopants during an annealing stage after dopant deposition. Again excimer laser annealing can be used for this purpose, or a so-called rapid thermal anneal process has been proposed. This process uses higher temperatures but for short durations of time. An optical heating source such as tungsten-halogen or Xe Arc lamp is often used as the heat source. This can however result in unwanted excessive heating of the glass substrate.
More recent publications have shown that polysilicon can be formed from amorphous silicon using an integrated metal heater element fabricated above the transistor structure. For example, reference is made to the paper “Polycrystalline Silicon Thin-Film Transistors Fabricated by Rapid Joule Heating Method” of Y. Kaneko et al, IEEE EDL, Vol 24, No. 9, p 586 (2003). Reference is also made to the paper “Rapid crystallization of silicon films using Joule heating of metal films” of T. Sameshima et al, App. Phys. A, A 73, p. 419-423 (2001).
In these proposals, a dielectric layer separates a thin film heater from the amorphous silicon. Current is passed through the heater element in pulses, for example 10 to 100 microseconds, which is sufficient to melt the underlying a-Si. However, the substrate remains cool, so that the process is effectively a low temperature process. The heaters are also used to activate implanted source and drain dopants.
These processes involve the treatment of the amorphous silicon layer by a thin film heating arrangement deposited over the amorphous silicon layer. After the heating process is completed, the layers defining the heating arrangement are removed, and the device is completed in conventional manner.